EPDM rubber, which is an elastic terpolymer of ethylene, an alpha-olefin such as propylene, etc., and a diene such as ethylidene norbornene, etc., has a molecular structure having no unsaturated bond in the main chain, and has superior weather resistance, chemical resistance and heat resistance to the general conjugated diene rubbers. Owing to these properties, the elastic terpolymers such as EPDM rubber have been used in a wide variety of industrial applications such as automotive part materials, electric wire materials, construction and hoses, gaskets, belts, bumpers, blends with plastics, etc.
Conventionally, the elastic terpolymers such as EPDM rubber have been mainly prepared by copolymerization of three kinds of monomers using a catalyst containing a vanadium compound, for example, a vanadium-based Ziegler-Natta catalyst. However, a large amount of the vanadium-based catalyst is needed because of its low catalytic activity, thereby causing a problem of increasing the content of the metal remaining in the copolymer. Thus, processes for catalyst removal and decolorization are required after preparation of the copolymer, and the residual catalyst in the polymer may cause deterioration of heat resistance, generation of impurities, inhibition of vulcanization, etc. Practically, when the elastic terpolymer is prepared using the catalyst containing the vanadium compound, it is difficult to control the reaction temperature due to the low polymerization activity and low temperature polymerization conditions, and also to control the molecular structure of the copolymer due to difficulties in the control of feeding amounts of comonomers such as propylene and a diene. Accordingly, there has been a limitation in the preparation of the elastic terpolymer having various physical properties by using the vanadium-based catalyst. Due to these problems, a method for preparing the elastic terpolymers such as EPDM rubber using a Group IV metallocene-based transition metal catalyst instead of vanadium-based Ziegler-Natta catalyst has been recently developed.
Since the Group IV transition metal catalyst exhibits high polymerization activity in the olefin polymerization, it is possible to prepare copolymers having a higher molecular weight, and also to easily control the molecular weight distribution and composition of the copolymer. In addition, the catalyst has an advantage that a variety of comonomers can be copolymerized. For example, U.S. Pat. Nos. 5,229,478 and 6,545,088, and Korean Patent No. 0488833 disclose that elastic terpolymers having a high molecular weight can be obtained with excellent polymerization activity by using various metallocene-based Group IV transition metal catalysts obtained from ligands such as cyclopentadienyl, indenyl, fluorenyl, etc.
However, when three kinds of monomers are copolymerized using these conventional Group IV transition metal catalysts, there is a disadvantage that distributions of the repeating units derived from the monomers are not uniform in the copolymer chains due to high reactivity for comonomers of alpha-olefins. As a result, it is difficult to obtain elastic terpolymers such as EPDM rubber having excellent elasticity and flexibility.
Further, U.S. Pat. No. 5,902,867 discloses a method for decreasing viscosity of the polymer by broadening of the molecular weight distribution in order to improve kneading processability and extrusion processability of EPDM. In this case, however, there is a limitation that polymer separation occurs during processing due to low molecular weight components included in the crosslinked rubber product, leading to deterioration of surface properties and low-temperature properties.
Accordingly, there is a continuous demand for a long-chain branched elastic terpolymer capable of satisfying excellent processability and elasticity (flexibility) at the same time, and a preparation method capable of preparing the same with high productivity and yield.